This invention relates to components for fiber optic telecommunication, specifically to such devices that can impart a partial polarization on the optical signal. The magnitude of this polarization can be adjusted.
Many components used in fiber optic telecommunications systems impart a partial polarization on the optical signal. This characteristic is referred to as polarization dependent loss (PDL). This is an undesirable property that can lead to amplitude modulation distortion in analog fiber optic systems, and increased bit error rate in digital fiber optic systems. The fiber optic components that possess a significant amount of PDL include couplers, optical switches, isolators, and wavelength division multiplexing filters. The problem is further complicated by the fact that the combined PDL of a concatenation of several components in a fiber optic link does not stay constant but instead fluctuates. This is due to the fact that the optical fiber connecting the components randomly transforms the polarization of the optical signal. This random transformation is affected by external conditions such as temperature and the stress on the fiber, which varies with time. It is due to this time variation that the combined PDLs of all the components are a coupled randomly over time, to produce a time-varying overall PDL.
In order to mitigate the deleterious effects of PDL, it is necessary to be able to accurately measure, simulate, and if necessary compensate for PDL. For all of these applications it is desirable to have a device that can produce a specified amount of PDL. Two additional requirements would be that the PDL remains constant over a specified wavelength range, and that it would be possible to deterministically vary the amount of PDL that this device can produce. For PDL measurement, this device can act as a calibration reference for the measuring instrument. For simulation of PDL, this device can be inserted in the fiber optic link, and the performance of the link can be monitored as the PDL is varied. For compensation of PDL, the device is inserted into the fiber optic link along with a polarization controller. The polarization controller is adjusted until the PDL axis of the device is perpendicular to the PDL axis of the composite PDL of the fiber link. The PDL of the PDL source is then adjusted until the overall PDL of the link is reduced to zero.
To date, several approaches have been used to produce sources of PDL in fiber optic systems. They include sections of polarizing fiber, tilting glass plates, and a variable polarization beamsplitter. These methods and their disadvantages will now be described:
(a) PDL can be produced by a section of polarizing fiber. By varying the length of the section, a different amount of PDL can be produced. This method however suffers from a strong variation of PDL with the wavelength. It is also difficult to predict a priori the exact PDL of a section of fiber. Finally, the availability of polarizing fiber is very limited since a major supplier of polarizing fiber (3M) has recently stopped producing it.
(b) PDL can be produced by a variable polarization beam splitter. This is a liquid crystal device that splits a beam into orthogonal polarizations, with each polarization exiting into a separate fiber. Whether the orthogonal polarizations are completely split or just partially split can be adjusted. Thus both of the two paths act like partial polarizers, whose degree of polarization can be adjusted. This device can therefore be used to produce PDL. The main disadvantage of this device is the high insertion loss (xcx9c1.5 dB) that results due to the use of liquid crystals. This device also requires a voltage to operate and thus cannot be adjusted manually.
(c) PDL can be produced by one or more tilting glass plates. The preferred embodiment of the invention is based on this principal. When a light beam is incident on a tilted glass plate, the s and p polarized light is transmitted with different efficiencies. The magnitude of transmission of each component is dependent on the tilt angle of the plate. Thus a varying amount of PDL can be produced. When just a single plate is used, the light beam is laterally offset by an amount proportional to the tilt angle. This lateral offset makes coupling the light beam back into a fiber more difficult. In addition, if an uncoated glass plate is used, the insertion loss becomes excessively high and accurate adjustment of PDL is difficult since a small change in the tilt angle will result in a large change in PDL.
Accordingly, several objects and advantages of the present invention are:
(a) to provide a fiber optic device with a variable amount of PDL which can be accurately adjusted manually or using an electric motor;
(b) to provide a fiber optic device with a variable amount of PDL, where the PDL does not vary significantly over the prescribed wavelength range of the input optical signal;
(c) to provide a fiber optic device with a variable amount of PDL where the insertion loss of the low loss axis of the device remains low.
(d) to provide a fiber optic device with a variable amount of PDL with compact dimensions.